Trophic structure of a macroarthropod litter food web in managed coniferous forest stands: a stable isotope analysis with δN and δC

We studied the composition of a litter detrital community in a temperate coniferous forest using stable isotopes of nitrogen and carbon. Samples of mineral soil, bulk litter material, macroarthropods and understory plants were collected from ten experimental forest stands. Half of the stands were previously thinned 17–42 years ago, the other half served as controls. Values of δ¹⁵N and δ¹³C were based on the analysis of almost 500 individuals of at least 22 species in 11 arthropod families. The isotopic analysis showed a significant increase in δ¹⁵N and δ¹³C values with soil depth. Isotopic signatures of macroarthropods ranged from −26.51‰ to −20.52‰ for δ¹³C and −2.85‰ to 5.10‰ for δ¹⁵N. All consumers showed levels of ¹³C enrichment substantially higher than those of primary producers and litter. Predators were generally significantly more ¹⁵N enriched than detritivores and herbivores, but their δ¹³C levels were similar to those of primary consumers. Our data indicate that this community consists of at least 2–3 trophic levels with a considerable amount of variation in the ¹⁵N enrichment among detritivores and predators. We suggest that the spread of δ¹⁵N values of predators likely reflects the diversity of potential prey among detritivores and a varying degree of intraguild predation among different species. Our findings generally agree closely with the results of similar studies from other forest litter communities. Thinning did not appear to influence the overall isotopic composition of the detrital food web. Extensive omnivory and intraguild predation among litter consumers may buffer long-term effects of thinning on the trophic structure of these species-rich communities.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Zea mays rhizosphere respiration, but not soil organic matter decomposition was stable across a temperature gradient

In a greenhouse experiment, we grew maize plants at different densities. We added fertilizer to half of the pots and created a temperature gradient. After 10 weeks of plant growth, we measured soil CO₂ efflux (SCE) and determined rhizosphere respiration (R_rhizo) and the decomposition rate of soil organic matter (R_SOM) using the different δ¹³C of the C₃ soil and C₄ plants. Whereas R_rhizo remained stable across the temperature gradient, R_SOM significantly increased with growth temperature. Neither plant density, nor the fertilizer treatment affected the relation between R_rhizo or R_SOM and growth temperature. Although R_rhizo might still increase with temperature in the short term, long term exposure to higher temperatures revealed full thermal acclimation of R_rhizo, but not of R_SOM.     

Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes

In agricultural systems with low S inputs, soil organic matter is a major source of S and the transformations between organic and inorganic S pools are important for the supply of S to plants. This study was conducted to determine the effect of S fertilizer on the size and activity of organic S pools. For 5 years S fertilizer with a known composition of stable S isotopes was applied to a rotation on a loamy soil and a coarse sandy soil at rates higher than the plant demand. Total organic S in soil organic matter was not affected by sulphur application, but a small increase occurred in the sulphate ester fractions (P<0.05). Inorganic sulphate concentrations in the soil reflected the S application in the year of sampling, whereas S applied in earlier years was not recognized. Organic matter below the plough layer in both soils was enriched with S, possibly as a result or organic matter leaching or an increased clay content in the subsoils. At 0–20 cm, the C:S ratio in organic matter was ca. 100 for both soils, decreasing to 73 and 46 at 60–80 cm for the coarse sandy soil and the loamy soils, respectively. In both soils, isotope data showed that ca. 30% of organic-bonded S at 0–20 cm originated from fertilizer S applied during the last 5 years, irrespective of the S application rate. At 20–40 cm the rate of incorporations was lower and at 40–60 cm no incorporation of fertilizer S into organic matter was recognized. The fertilizer application did not induce net changes in the total organic S fraction, but isotope data indicated that a considerable part of the organic S pool was involved in S cycling in the field.     

Changes in soil organic carbon dynamics in an Eastern Chinese coastal wetland following invasion by a C4 plant Spartina alterniflora

The exotic C₄ grass Spartina alterniflora was intentionally introduced to tidal coastal wetlands in Jiangsu province of China in 1982. Since then it has rapidly replaced the native C₃ plant Suaeda salsa, becoming one of the dominant vegetation types in the coastal wetlands of China. Although plant invasion can change soil organic carbon (SOC) storage, little is known about how plant invasion influences C storage within soil fractions. We investigated how S. alterniflora invasion across an 8, 12 and 14-year chronosequence affected SOC and soil nitrogen (N), using soil fractionation and stable δ¹³C isotope analyses. SOC and N concentrations at 0-10 cm depth in S. alterniflora soil increased during the S. alterniflora invasion chronosequence, ranging from 3.67 to 4.90 g C kg⁻¹ soil, and from 0.307 to 0.391 g N kg⁻¹ soil. These were significantly higher than the values in the Suaeda salsa community, by 27.0-69.6% for SOC, and 21.8-55.2% for total N. The S. alterniflora-derived SOC varied from 0.40 to 0.92 g C kg⁻¹ according to mixing calculations, assuming the two possible SOC sources of S. alterniflora and S. salsa, and accounted for 10.8-18.7% of total SOC in the colonized soils. The estimated accumulative rate of SOC from C₄ (S. alterniflora) was 64.1 C kg⁻¹ soil year⁻¹ and from C₃ sources was 78.1 mg C kg⁻¹. The concentration of S. alterniflora-derived SOC significantly decreased from coarse fraction to fine fraction, and linearly increased as the period of S. alterniflora invasion increased. The highest accumulative rate of SOC from a C₄ source occurred in macroaggregates, while the highest rate from C₃ was in microaggregates. The storage of SOC derived from S. alterniflora in the macroaggregates was 0.27-0.44 g C kg⁻¹ soil, accounting for 43.1-49.1% of the total C₄derived SOC in the soil. Our results suggest that S. alterniflora invasion in coastal wetlands could facilitate SOC storage, because of the high potential for accumulation of the C which has been newly derived from S. alterniflora litter and roots.     

Effects of past, present and future atmospheric CO2 concentrations on soil organic matter dynamics in a chaparral ecosystem

Owing to the continuously increasing concentration of atmospheric CO₂, it has become a priority to understand if soil organic matter (SOM) will behave as a sink or a source of CO₂ under future environmental changes. Although many studies have addressed this question, a clear understanding is still missing, particularly with respect to long-term responses. In this study, we quantified soil C stores and dynamics in relationship to soil aggregation and pool composition in a Californian chaparral ecosystem exposed for 6 years to a gradient of atmospheric CO₂ concentrations, ranging from pre-industrial levels 250 to 750 μl l⁻¹ CO₂. Fossil fuel-derived CO₂ depleted in ¹³C was used for the fumigation, thus providing a tracer of C input from the vegetation to the soil.Long-term CO₂ exposure invariably affected soil aggregation, with a significant decrease in the macroaggregate fraction at highest CO₂ levels relative to the other two size fractions (i.e. microaggregates and silt and clay). This soil structural change most likely reduced the stability and protection of SOM, and C content generally decreased in most fractions over the CO₂ treatments, and induced faster turnover of recently fixed C at high CO₂ levels. The strongest response was found in the C content of the microaggregates, which decreased significantly (P<0.05) with rising levels of CO₂. We conclude that increasing atmospheric CO₂ concentrations will decrease soil C in chaparral ecosystems, and that the microaggregate fraction is the most responsive to increasing concentrations of atmospheric CO₂.     

Thermal stability of soil organic matter pools and their δC values after C3-C4 vegetation change

Carbon isotopic composition of soils subjected to C₃-C₄ vegetation change is a suitable tool for the estimation of C turnover in soil organic matter (SOM) pools. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability. Soil samples from a field plot with 10.5 years of cultivation of the C₄ plant Miscanthus×gigantheus and from a reference plot under C₃ grassland vegetation were analysed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). According to differential weight losses (dTG) and energy release or consumption (DSC), five SOM pools with increasing thermal stability were distinguished: (I) 20-190 °C, (II) 190-310 °C, (III) 310-390 °C, (IV) 390-480 °C, and (V) 480-1000 °C. Their δ¹³C values were analysed by EA-IRMS. The weight losses in pool I were connected with water evaporation, since no significant C losses were measured and δ¹³C values remained unchanged. The δ¹³C of pools II and III in soil samples under Miscanthus were closer to the δ¹³C of the Miscanthus plant tissues (−11.8‰) compared to the thermally stable SOM pool V (−19.5‰). The portion of the Miscanthus-derived C₄-C in total SOM in 0-5 cm reached 55.4% in the 10.5 years. The C₄-C contribution in pool II was 60% and decreased down to 6% in pool V. The mean residence times (MRT) of SOM pools II, III, and IV were similar (11.6, 12.2, and 15.4 years, respectively), while pool V had a MRT of 163 years. Therefore, we concluded that the biological availability of thermal labile SOM pools (<480 °C) was higher, than that of the thermal stable pool decomposed above 480 °C. However, the increase of SOM stability with rising temperature was not gradual. Therefore, the applicability of the TG-DSC for the separation of SOM pools with different biological availability is limited.     

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO₂ alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO₂ induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO₂. However, the long turnover times of many SOC pools - decades to centuries - make the detection of changes in the soil's pool size difficult. Long-term CO₂ enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO₂ has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO₂; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO₂ influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO₂. Specifically, we need to understand how elevated CO₂ influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.     

Long-term soil organic carbon dynamics in a subhumid tropical climate: C data in mixed C3/C4 cropping and modeling with ROTHC

Scanty information on long-term soil organic carbon (SOC) dynamics hampers validation of SOC models in the tropics. We observed SOC content changes in a 16-year continuously cropped agroforestry experiment in Ibadan, south-western Nigeria. SOC levels declined in all treatments. The decline was most pronounced in the no-tree control treatments with continuous maize and cowpea cropping, where SOC levels dropped from the initial 15.4 to 7.3-8.0 Mg C ha⁻¹ in the 0-12 cm topsoil in 16 years. In the two continuously cropped alley cropping (AC) systems, one with Leucaena leucocephala and one with Senna siamea trees, SOC levels dropped to 10.7-13.2 Mg C ha⁻¹. Compared to the no-tree control treatments, an annual application of an additional 8.5 Mg ha⁻¹ (dry matter) of plant residues, mainly tree prunings, led to an extra 3.5 Mg C ha⁻¹ (∼0.2% C) in the 0-12 cm top soil after 11 years, and 4.1 Mg C ha⁻¹ after 16 years. The addition of NPK fertilizer had little effect on the quantities of above-ground plant residues returned to the soil, and there was no evidence that the fertilizer affected the rate of SOC decomposition. The fact that both C₃ and C₄ plants returned organic matter to the soil in all cropping systems, but in contrasting proportions, led to clear contrasts in the ¹³C abundance in the SOC. This ¹³C information, together with the measured SOC contents, was used to test the ROTHC model. Decomposition was very fast, illustrated by the fact that we had to double all decomposition rate constants in the model in order to simulate the measured contrasts in SOC contents and δ¹³C between the AC treatments and the no-tree controls. We hypothesized (1) that the pruning materials from the legume trees and/or the extra rhizodeposition from the tree roots in the AC treatments accelerated the decomposition of the SOC present at the start of the experiment (true C-priming), and/or (2) that the physical protection of microbial biomass and metabolites by the clay fraction on this site, having a sandy top soil in which clay minerals are mainly of the 1:1 type, is lower than assumed by the model.     

Turnover of soil organic matter and of microbial biomass under C3-C4 vegetation change: Consideration of C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ¹³C signature in soil pools: ¹³C fractionation per se and preferential microbial utilization of various substrates with different δ¹³C signature. These two processes were disentangled by simultaneously tracking δ¹³C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO₂ efflux during incubation of 1) soil after C₃-C₄ vegetation change, and 2) the reference C₃ soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C₃ vegetation. Miscanthus giganteus - a perennial C₄ plant - was grown for 12 years, and the δ¹³C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ¹³C signature of the three C pools and of CO₂ in the reference C₃ soil were less than 1‰, and only δ¹³C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ¹³C of all pools in the soil after C₃-C₄ vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO₂. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ¹³C changes in DOC, CO₂ and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ¹³C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ¹³C values) and not ¹³C fractionation per se.Based on the δ¹³C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ¹³C signature of linked pools helps calculate the relative turnover of old and recent pools.     

Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration (R_s), soil temperature (T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R_s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO₂ efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO₂ efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m⁻² s⁻¹ in the forest, abandoned field and olive grove, respectively. R_s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0-15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R_s was controlled by soil moisture. Exponential and linear models adequately described R_s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R_s in the forest and olive grove. However, in the abandoned field, a single variable - either soil temperature (growing season) or rewetting index (dry season) - was sufficient to explain between 51 and 63% of the soil CO₂ efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO₂ efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.     

Short-term C4 plant Spartina alterniflora invasions change the soil carbon in C3 plant-dominated tidal wetlands on a growing estuarine Island

Spartina alterniflora is an invasive C₄ perennial grass, native to North America, and has spread rapidly along the east coast of China since its introduction in 1979. Since its intentional introduction to the Jiuduansha Island in the Yangtze River estuary, Spartina alterniflora community has become one of the dominant vegetation types. We investigated the soil carbon in the Spartina alterniflora community and compared it with that of the native C₃Scirpus mariqueter community by measuring total soil carbon (TC), soil organic carbon (SOC), total soil nitrogen (TN), and the stable carbon isotope composition (δ¹³C) of various fractions. TC and SOC were significantly higher in Spartina alterniflora in the top 60 cm of soil. However, there was no significant difference in soil inorganic carbon (IC) between the two communities. Stable carbon isotopic analysis suggests that the fraction of SOC pool contributed by Spartina alterniflora varied from 0.90% to 10.64% at a soil depth of 0-100 cm with a greater percentage between 20 and 40 cm deep soils. The δ¹³C decreased with increasing soil depth in both communities, but the difference in δ¹³C among layers of the top 60 cm soil was significant (p<0.05), while that for the deeper soil layers (>60 cm) was not detected statistically. The changes in δ¹³C with depth appeared to be associated with the small contribution of residues from Spartina alterniflora at greater soil depth that was directly related to the vertical root distribution of the species.     

Disruption of soil aggregates by varied amounts of ultrasonic energy in fractionation of organic matter of a clay Latosol: carbon,nitrogen and δ13C distribution in particle‐size fractions

Ultrasonic energy has been widely used to disrupt soil aggregates before fractionating soil physically when studying soil organic matter (SOM). Nevertheless, there is no consensus about the optimum energy desirable to disrupt the soil. We therefore aimed (i) to quantify the effect of varied ultrasonic energies on the recovery of each particle‐size fraction and their C, N and δ¹³C distribution, and (ii) to determine an ideal energy to fractionate SOM of a specific soil. Our results show that the 2000–100 μm particle‐size fraction was composed mainly of unstable aggregates and the 100–2 μm fraction of stable aggregates. Energies of 260–275 J ml^?1 were sufficient to disrupt most of the unstable aggregates and leave stable aggregates. The use of this threshold energy combined with particle‐size fractionation was not satisfactory for all purposes, since litter‐like material and relatively recalcitrant organic carbon present in stable aggregates > 100 μm were recovered in the same pool. An ultrasonic energy of 825 J ml^?1 was not sufficient to stabilize the redistribution of soil mass and organic matter among particle‐size fractions, but at energies exceeding 260–275 J ml^?1 relatively stable aggregates would fall apart and cause a mixture of carbon with varied nature in the clay fraction.     

The effects of compost in a rice–wheat cropping system on aggregate size,carbon and nitrogen content of the size–density fraction and chemical composition of soil organic matter,as shown by 13C CP NMR spectroscopy

We investigated whether the long‐term application of compost from agricultural waste improved soil physical structure, fertility and soil organic matter (SOM) storage. In 2006, we began a long‐term field experiment based on a rice–wheat rotation cropping system, having a control without fertilizer (NF) and three treatments: chemical fertilizers (CF), pig manure compost (PMC) and a prilled mixture of PMC and inorganic fertilizers (OICF). Following the harvest of wheat in 2010, the mean‐weight diameter (MWD) of water‐stable aggregates and the concentration of C and N in bulk soil (0–20 cm; <2 mm fraction) were significantly greater (P < 0.05) in PMC and NF plots than in CF or OICF plots. Pig manure compost significantly increased the proportion of >5‐mm aggregates, whereas CF significantly increased the proportion of 0.45‐ to 1‐mm aggregates. The C and N contents of all density fractions were greater in PMC than in other treatments with levels decreasing in the following order: free particulate organic matter (fPOM) >occluded particulate organic matter (oPOM) > mineral‐combined SOM (mineral–SOM). Solid‐state ¹³C CPMAS NMR spectra showed that alkyl C/O‐alkyl C ratios and aromatic component levels of SOM were smaller in PMC and OICF plots than in CF plots, suggesting that SOM in PMC and OICF plots was less degraded than that in CF plots. Nevertheless, yields of wheat in PMC and NF plots were smaller than those in CF and OICF plots, indicating that conditions for producing large grain yields did not maintain soil fertility.     

Changes in the fungal-to-bacterial respiratory ratio and microbial biomass in agriculturally managed soils under free-air CO2 enrichment (FACE) - A six-year survey of a field study

In soil ecology, microbial parameters have been identified as sensitive indicators of changes in the soil environment. The Braunschweig FACE project provided the opportunity to study the effects of elevated CO₂ (550 μmol mol⁻¹) as compared to ambient CO₂ (370 μmol mol⁻¹) on total microbial biomass (C_mic), C_mic-to-C_org ratio and the fungal-to-bacterial respiratory ratio together with total C_org, N_t, C:N ratio and pH over a six-year period. Field management followed a typical crop rotation system of this region with either a crop-related full nitrogen supply (N100) or 50% reduced N supply (N50). The soil microbial parameters responded to the elevated CO₂ treatment in varying intensities and time spans. The fungal-to-bacterial respiratory ratio was the most sensitive parameter in responding to an elevated CO₂ treatment with highly significant differences to ambient CO₂-treated control plots in the third year of CO₂ fumigation. After six years bacterial respiratory activity had increased in ascending order to 34% in FACE-treated plots (N50 and N100) as compared to control plots. Soil microbial biomass (C_mic) responded more slowly to the FACE treatment with highly significant increases of >12% after the fourth year of CO₂ fumigation. The C_mic-to-C_org ratio responded very late in the last two years of the CO₂ treatment with a significant increase of >7.0% only in the N100 variant. Total C_org and N_t were slightly but significantly increased under FACE around 10.0% with ascending tendency over time starting with the second year of CO₂ treatment. No significant FACE effects could be recorded for the C:N ratio or pH.These results suggest that under FACE treatment changes in the soil microbial community will occur. In our study the fungal-to-bacterial respiratory ratio was superior to total C_mic as microbial bioindicators in reflecting changes in the soil organic matter composition.